A multi-channel optical link is an optical link designed to convey optical signals over a plurality of channels, each channel having a transmission wavelength that is distinct from those of the other channels. When the optical link is both-way, it conveys a plurality of channels in both transmission directions, commonly referred to as the "go" direction and the "return" direction.
Currently envisaged multi-channel optical links are constituted by lines that are optically continuous over very long distances (several tens of thousands of kilometers). In that type of link, a traffic optical signal is never conveyed from one end of the link to the other. In general, optical channels are dropped and/or inserted at regular intervals along the link for the purposes of serving particular geographic zones. Such dropping and inserting is performed by means of drop/insert apparatus comprising optical circulators and diffraction gratings such as Bragg gratings.
Conventional channel-dropping apparatus as used to drop a channel from a one-way optical link comprises a three-port optical circulator inserted directly in the line optical fiber so that two of its ports are connected thereto. Its third port is connected to an "auxiliary" optical fiber portion. Finally, a diffraction grating is inserted in the line optical fiber, e.g. downstream from the circulator if said circulator operates clockwise.
That apparatus operates as follows for an optical link which is capable of conveying n channels (where n is an integer not less than 2) and from which a channel of wavelength .lambda.i (where i lies in the range 1 to n) is to be dropped to serve a given geographic zone (assuming that all of the channels are present on the link on arriving at the dropping apparatus).
Signals of wavelength .lambda.j (where j lies in the range 1 to n) arrive at the circulator via its first port, and they are then directed to its second port which directs them to the diffraction grating. The diffraction grating has a reflection wavelength equal to .lambda.i. Signals of wavelength .lambda.i are thus reflected by the diffraction grating towards the second port of the circulator which port then directs them towards the auxiliary optical fiber, the other end of which is connected, for example, to an optical signal receiver. Signals of wavelength .lambda.i (and therefore the channel .lambda.i) are thus dropped from the link.
Such apparatus could be used to insert a channel of wavelength .lambda.k into the link in the return direction via the auxiliary fiber by means of the circulator which would direct it directly in the return direction. Thus, such apparatus could serve to drop a channel in the go direction and to insert a channel in the return direction.
It would thus be reasonable to think that by using such apparatus associated with symmetrical apparatus (a circulator operating counterclockwise associated with a diffraction grating inserted in the line fiber upstream from the circulator) would enable channels to be dropped and inserted in both directions of the both-way link.
Unfortunately, that is not the case. If two symmetrical apparatuses of that type are associated, it is not possible for a channel whose wavelength is identical to that of a channel that has been dropped in the go direction to be conveyed over the link in the return direction, because of the diffraction grating inserted in the line fiber to perform dropping in the go direction. Similarly, it is not possible for a channel whose wavelength is identical to that of a channel that has been dropped in the return direction to be conveyed over the link in the go direction, because of the diffraction grating inserted in the line fiber to perform dropping in the return direction.
It can thus be well understood that, although such a solution appears simple initially, it is very inflexible.